A top Iranian official, in an unusual declaration Saturday, said there remains no trust between Tehran and world powers and either side could yet abandon a nuclear deal after signing.

In comments that laid bare a paradox of long-running negotiations between Iran and the West, Deputy Foreign Minister Abbas Araghchi said an agreement -- due by June 30 -- was nearing despite neither side trusting the other.

A final accord could rein in aspects of Iran's nuclear programme while allowing it to continue to enrich uranium in exchange for a lifting of sanctions, but breaches could see the deal being ripped up.

"Our basis is mistrust and this is the reality," Araghchi was quoted by state television as saying at the end of the latest round of talks in Vienna with the P5+1 group of nations that has been talking to Iran for almost two years.

"We don't trust the other side at all and they don't trust us either," Araghchi said, noting so-called "snapback" provisions would be as relevant to Iran as to the United States if measures are reneged on.

"Thus all the provisions in a deal... whenever each party feels the other side is violating the commitments, they can snap back and implement whatever existed before the agreement.

"We have taken every necessary measure so this would happen for us. Naturally, the other side will do the same for sanctions," he added.

Araghchi, a key figure in the talks, also revealed the text of a final agreement would comprise a main document of about 20 pages accompanied by five appendices totalling a further 40 to 50 pages.

"Each word of this instrument is being discussed and sometimes quarrelled on," he said. "There are differences but work moves forward very slowly."

One appendix would be related to sanctions to be lifted on Iran, while others would document the technical limits of Iran's nuclear programme, allowable research and development and a "common committee" that would supervise the deal, he said.

The final appendix would lay out the timetable for the likely 10-year agreement "showing by when the two sides have to take the required measures," according to Araghchi.

Iran maintains its nuclear programme has no military implications and is solely for peaceful purposes -- a contention western powers have long disputed.

Recent weeks have seen arguments on how Iran's nuclear activities can be monitored.

International inspections of Iran's atomic facilities will be a cornerstone of any deal but officials in Tehran have said military sites would be off-limits.

On April 2, Iran and the P5+1 -- the United States, Britain, China, France, Russia plus Germany -- agreed the main outlines of the nuclear deal, with Tehran agreeing to mothball large sections of its atomic programme.

With the final deadline approaching both the US and Iran are under immense pressure from respective hardliners not to make further concessions.